1. Field of the Invention
The present invention relates generally to tuner circuits and, more particularly, is directed to a tuner circuit for use with a color television receiver for receiving, for example, a CATV broadcast or a tuner circuit for use with a videotape recorder for recording the CATV broadcast.
2. Description of the Prior Art
Recently, with the spread of CATV broadcast, many broadcast channels from VHF band to UHF band have become available, for example, in the United States of America. Frequently color television receivers and the videotape recorder are designed to have a tuner circuit which is capable of receiving these broadcasts ranging from VHF low band to the UHF band.
The above-mentioned tuner circuit is generally formed so as to receive 4 band signals (UHF band and VHF 3 bands). The VHF 3 band portions are sub-divided to provide the lowest frequency band, the highest frequency band and the intermediate frequency band. The lowest frequency band will hereinafter be referred to as a VL band and covers from, for example, on Air broadcast 2CH (55.25 MHz) to CATV A-3CH broadcast (103.25 MHz). The highest frequency band will hereinafter be referred to as a VH band and covers from, for example, CATV JCH (217.25 MHz) to CATV WWCH (430 MHz). The intermediate frequency band will hereinafter be referred to as a VM band and covers from, for example, CATV A-2CH (109.25 MHZ) to on Air broadcast 13CH (211.25 MHz).
Thus, the high frequency amplifier stage is formed of two systems which comprise, as shown in FIG. 1, a high frequency amplifying stage 1 for receiving VL and VM band signals and a high frequency amplifying stage 2 for receiving a VH band signal. The reason for this is that when the three band signals are received by a single common high frequency (HF) amplifier stage, it is difficult to obtain good frequency characteristics over the whole bands. In FIG. 1, reference numeral 3 designates a high frequency amplifying stage which receives a UHF band signal.
Recently, in order to miniaturize and reduce the tuner circuit from a size and money standpoint, it has been proposed to receive three VL, VM and VH band signals by a single common high frequency (HF) amplifier stage 4 as shown in FIG. 2.
FIG. 3 illustrates a practical circuit arrangement of the above-mentioned single common high frequency amplifier stage shown in FIG. 2. In FIG. 3, a .lambda./4 type of resonant circuit is employed. The fundamental circuit arrangement of the .lambda./4 type resonant circuit is represented in FIG. 4. Use of the nomenclature relating to .lambda./4 and .lambda./2 for resonant circuits in high-frequency tuners is well-known and is based upon distributed constants in a length of coaxial cable. FIGS. 4 and 6 show the lumped constant equivalent circuits for these resonant circuits. Thus, the circuits referred to as .lambda./4 and .lambda./2 correspond in function to the known distributed constant, coaxial cable resonant circuits. As shown in FIG. 4, this fundamental circuit comprises coil L and a capacitor C. In this case, its resonant frequency f.sub.0 is expressed by the following equation ##EQU1## where L and C are respectively the inductance and the capacitance of the coil and the capacitor.
Referring back to FIG. 3, when receiving the VL band signal, band switching signals "1" (for example, about 9V), "0" (for example, about 0.2 to 0V) and "0" are respectively supplied to band switching terminals 10, 11 and 12 from the external apparatus. A capacitor C1, a variable capacitance diode D1, a capacitor C2 and coils L2, L3 and L6 constitute a .lambda./4 resonant circuit of the primary side. A capacitor C4, a variable capacitance diode D6, a capacitor C3 and coils L4, L5 and L6 constitute a .lambda./4 type resonant circuit at the secondary side. A combined capacitance of the variable capacitance diode Dl and the capacitors C1, C2 at the primary side and a combined capacitance of the variable capacitance diode D6 and the capacitors C3, C4 at the secondary side each corresponds to the capacitor C shown in FIG. 4. The coil L in FIG. 4 corresponds to a combined inductance of the coils L2, L3 and L6 at the primary side and to a combined inductance of the coils L4, L5 and L6 at the secondary side.
Referring to FIG. 3, it will be seen that an input signal from an input resonant circuit (not shown) is supplied through a coupling capacitor C.sub.0 to a high frequency amplifier 14 formed of a dual gate type metal oxide semiconductor field effect transistor (MOSFET), thereby being amplified. The output of the MOSFET 14 is supplied through a varaible capacitance diode D7 to the primary side of the resontant circuit. The coupling factor of the variable capacitance diode D7 might be varied in response to the frequency. Also, the primary side of the resonant circuit is supplied with a tuning voltage through an input terminal 13 from a channel selecting circuit (not shown). Thus, the tuning of the primary side is effected by varying the capacitance of the variable capacitance diode Dl in the mixed capacitance of the variable capacitance diode Dl and the capacitors C1, C2. On the other hand, the tuning of the secondary side is effected by varying the capacitance of the variable capacitance diode D6 of the mixed capacitance of the variable capacitance diode D6 and the capacitors C3, C4. Then, a desired signal tuned to a predetermined tuned frequency is supplied to a mixing circuit (not shown) from the secondary side of the resonant circuit. A variable capacity diode D8 is used to change the coupling factor of the secondary side of the resonant circuit and the mixing circuit (not shown) in response to the frequency.
When the VM band signal is received, the band switching signals of "0", "1" and "0" are respectively supplied to the band switching terminals 10, 11 and 12 from the external apparatus, whereby the diodes D3 and D4 are turned on. Thus the mixed capacitance of the capacitor C1, the variable capacitance diode D1, the capacitor C2 and a capacitor C2' and the inductance of the coil 2 form the primary side of the .lambda./4 type resonant circuit, and the mixed capacitance of the capacitor C4, the variable capacitance diode D6, the capacitor C3 and a capacitor C3' and the inductance of the coil L4 forming the secondary side of the .lambda./4 type resonant circuit. At that time, in the primary side, the variable capacitance diode D1, the capacitors C1, C2 and C2' correspond to the capacitor C shown in FIG. 4, while in the secondary side, the varible capacitance diode D6, the capacitors C3, C3' and C4 correspond to the capacitor C shown in FIG. 4. Also, in the primary side, the coil L2 corresponds to the coil L of FIG. 4, while in the secondary side, the coil L4 corresponds to the coil L of FIG. 4. The tuning is carried out in the same way as that in which the tuning is carried out when the VL band signal is received.
When the VH band signal is received, the band switching signals of "0", "0" and "1" are respectively supplied to the band switching terminals 10, 11 and 12, turning the diodes D2 and D5 on. Thus, mainly the capacitor C1, the variable capacitance diode D1, the capacitor C2, the coil L1 and the capacitor C2" constitute the .lambda./4 type resonant circuit of the primary side, whereas mainly the capacitor C4, the variable capacitance diode D6, the capacitor C3, a coil L7 and a capacitor C3" constitute the .lambda./4 type resonant circuit of the secondary side. At that time, in the primary side, mainly the variable capacitance diode D1 and the capacitors C1, C2 and C2" correspond to the capacitor C shown in FIG. 4, while in the secondary side, mainly the variable capacitance diode D6 and the capacitors C3, C3" and C4 correspond to the capacitor C shown in FIG. 4. Also, mainly the coil L1 in the primary side corresponds to the second coil L in FIG. 4 and mainly the coil L7 in the secondary side corresponds to the coil L in FIG. 4. The inductances of the coils L1 and L7 are small so that when the VH band signals are received, a diode D9 is turned on and the diodes D3 and D4 are also turned on, whereby the coil L2 is connected in parallel to the coil L1 and the coil L4 is connected in parallel to the coil L7, thereby compensating the small inductances of the coils L1 and L7. Further, the tuning is effected in the same way as in the case of the VL and VM band signals.
In FIG. 3, reference numeral CH1 designates a choke coil, R1 an overcurrent preventing resistor and R2 to R6 the bias resistors. The remaining other resistors are all used as the bias resistors.
In case of the circuit system shown in FIG. 3, the inductances of the coils L1 and L7 are not selected so large so that when the VH band signals are received, the circuit loss is increased.
To remove the above-mentioned defects, it is proposed to construct the circuit by a .lambda./2 type resonant circuit shown in FIG. 5. FIG. 6 shows the fundamental circuit of the .lambda./2 type resonant circuit which comprises a coil L and capacitors C' and C". Assuming that L and C', C" represent the inductance and the capacitances thereof, too, then its resonant frequency f'.sub.0 is expressed by the following equation. ##EQU2## If C=C'=C" is established, the inductance L has to be doubled in order to obtain the same resonant frequency f'.sub.0. In other words, the larger the L/C becomes the higher the Q becomes, thus the circuit loss becomes small.
Turning back to FIG. 5, when the VL band signals are received, the band switching signals "1", "0" and "0" are respectively supplied to the band switching terminals 10, 11 and 12 from the external apparatus. Then, the capacitor C5, the variable capacitance diode D10, the coil L8, the variable capacitance diode D11, the capacitor C6, the coils L9, L10 and L11 constitute the .lambda./2 type resonant circuit of the primary side, whereas the capacitor C7, the variable capacitance diode D12, the coil L12, the variable capacitance diode D13, the capacitor C8, and the coils L13, L14 and L11 constitute the .lambda./2 type resonant circuit of the secondary side. In that event, to the capacitors C' and C" shown in FIG. 6, in the primary side, there correspond the capacitor C5 and the variable capacitance diode D10 and, the variable capacitance diode D11 and the capacitor C6, while in the secondary side, these correspond the capacitor C8 and the variable capacitance diode D13 and the varable capacitance diode D12 and the capacitor C7, respectively. Further, the coils L8, L9, L10 and L11 of the primary side correspond to the coil L shown in FIG. 6, and the coils L12, L13, L14 and L11 of the secondary side correspond to the coil L shown in FIG. 6.
Turning back to FIG. 5, the input signal from the input tuning circuit is supplied to a dual gate type MOSFET 14, in which it is amplified and is then fed to the primary side of the resonant circuit. The tuning at the primary side is effected by varying the capacitances of the variable capacitance diodes D10 and D11, while the tuning at the secondary side is effected by varying the capacitances of the variable capacitance diodes D12 and D13 by a tuning voltage from the input terminal 13. Then, a desired signal tuned to a predetermined frequency is supplied to the mixing circuit from the secondary side of the resonant circuit.
When the VM band signals are received, the band switching signals "0", "1" and "0" are respectively supplied to the band switching terminals 10, 11 and 12, whereby the diodes D14 and D15 are turned on. Thus, the capacitor C5, the variable capacitance diode D10, the coil L8, the variable capacitance diode D11, the capacitor C6, the coil L9 and the capacitor C6' constitute the .lambda./2 type resonant circuit of the primary side. Further, the capacitor C7, the variable capacitance diode D12, the coil L12, the variable capacitance diode D13, the capacitor C8, the coil L13 and the capacitor C8' constitute the .lambda./2 type resonant circuit of the secondary side. At that time, the capacitors C' and C" shown in FIG. 6 correspond respectively to the capacitor C5 and the variable capacitance diode D10 and to the variable capacitance diode D11 and the capacitors C6, C6' at the primary side, and correspond respectively to the capacitors C8, C8' and the variable capacitance diode D13 and to the variable capacitance diode D12 and the capacitor C7 at the secondary side. The coil L of FIG. 6 corresponds to the coils L8 and L9 at the primary side and corresponds to the coils L12 and L13 at the secondary side. The tuning is effected similarly to that of the VL band signals.
When the VH band signals are received, the band switching signals of "0", "0" and "1" are respectively supplied to the band switching terminals 10, 11 and 12 from the external circuit, whereby the diodes D16 and D17 are turned on. Thus, mainly the capacitor C5, the variable capacitance diode D10, the coil L8, the variable capacitance diode D11 and the capacitors C6, C6" constitute the .lambda./2 type resonant circuit of the primary side. On the other hand, mainly the capacitor C7, the variable capacitance diode D12, the coil L12, the variable capacitance diode D13 and the capacitors C8, C8" constitute the .lambda./2 type resonant circuit of the secondary side. In that event, the capacitors C' to C" shown in FIG. 6 correspond respectively to the capacitor C5 and the variable capacitance diode D10 and to the variable capacitance diode D11 and the capacitors C6, C6" at the primary side, and correspond respectively to the capacitors C8, C8" and the variable capacitance diode D13 and to the variable capacitance diode D12 and the capacitor C7 at the secondary side. The coil L shown in FIG. 6 corrresponds to the coil L8 at the primary side and to the coil L12 at the secondary side.
When receiving the VH band signals, the diode D18 is turned on and the diodes D14 and D15 are turned on, whereby the coil L9 is serially connected to the coil 18 and the coil L13 is serially connected to the coil L12 thereby compensating for the inductances thereof. The tuning is effected similarly to those of the VL and VM band signals.
In case of the circuit system shown in FIG. 5, the inductances of the coils L8 and L12 corresponding to the coils L1 and L7 of the circuit shown in FIG. 3 can be selected to be large so that while the circuit loss when the VH band signals are received is small, many circuits are commonly used for the respective band signals. Thus, it is difficult to balance the characteristics of the resonant circuits among the respective band signals.